Add role_invoices

This new role allows users to create invoices without role_finance. Users
can then only modify their own created invoices until somebody with the
role_finance sets the paid_on value.

app/models/user.rb

@@ -143,7 +143,12 @@ class User < ActiveRecord::Base
   def role_finance?
     groups.detect {|group| group.role_finance?}
   end
-  
+
+  # Checks the invoices role
+  def role_invoices?
+    groups.detect {|group| group.role_invoices?}
+  end
+
   # Checks the article_meta role
   def role_article_meta?
     groups.detect {|group| group.role_article_meta?}